The screen shows 78 and your hand is already on the spade. It shows 12 and you walk on. That's roughly how the first month goes for most people, and roughly how half the finds get left behind.
VDI (Visual Discrimination Indicator) is an estimate of a target's conductivity — a hint. Not a passport, not a guarantee, and certainly not a diagnosis. Here's why.
What the machine is actually measuring
The detector evaluates how a target responds to an electromagnetic field and turns that into a number or a position on a scale. Low values tend towards iron and foil, mid and high values towards non-ferrous metals.
"Tend" is the operative word. The same coin reads differently at different depths, in different soil and at different angles. And the scales don't match between manufacturers: Equinox, Manticore, Deus II and Legend speak different dialects. Which is why somebody else's chart from a group chat is useless — the machine you need to learn is yours.
What discrimination is for
Discrimination mutes or removes part of the scale so you dig less rubbish. That sounds like pure upside, but every version of it has a price.
| Approach | Upside | Downside |
|---|---|---|
| Heavy iron discrimination | Fewer caps and nails | You cut masked non-ferrous, and useful information about nearby iron |
| Mild discrimination or all-metal | More data about the target | More holes, requires discipline |
| Notching one band | Removes a familiar trash peak | Easy to notch out coins in the same range |
The field rule is simple: stable ground and a readable tone first, fine discrimination second. A day that starts with "notch everything below forty" usually ends that way too.
Five reasons the number lies
- Depth. A weak signal drifts across the scale.
- Adjacent iron. The non-ferrous target gets masked and the VDI jumps.
- Shape and angle. A ring on edge and a coin lying flat are completely different responses.
- Mineralisation and salt. Beaches and red soil shift the ID; that's exactly why you read the Manticore's ID Map rather than a single digit.
- Caps and bi-metal. The classic impostors. Ferrocheck helps on the Legend, but a second sweep and a pinpointer help on everything.
How to learn
- Run a mode where iron is still audible, or keep discrimination mild.
- On a test garden, sweep a nail, a bottle cap, a copper coin and some silver. Memorise the tone, not the number.
- In the field, check the target from several directions and finish with the pinpointer.
- Log the VDI and mode next to a photo of the find. After twenty outings you'll have your own chart — the only one worth trusting.
All Metal or discriminate
On new or heavily trashed ground, more information is better: all-metal or low discrimination, so you understand what you're dealing with. On clean pasture worked for coins, moderate iron discrimination saves time and energy.
On a beach the priority is different — first get the machine to stop reacting to salt, and only then touch discrimination.
Three myths
"VDI 80 always means silver." It doesn't. It's a range, and plenty of things land in it.
"Nobody digs iron." People do: non-ferrous hides next to iron, and sometimes the iron object itself is a buckle or a fitting that dates the site.
"Higher discrimination means a more experienced detectorist." If anything, the reverse: usually it means somebody can't hear half the scale and chose to switch it off.
Before you head out
- I know my machine's scale, at least roughly.
- Discrimination isn't floored until the ground is stable.
- The decision to dig comes from tone and repeatability, not one number.
- Pinpointer on the belt.
- Doubtful targets go in the diary with the VDI and a note.